SOI wafer and its manufacturing method

ABSTRACT

Since a supporting wafer contains boron of 9×10 18  atoms/cm 3  or more, therefore a part of the metal impurities in an active layer wafer and the metal impurities in the wafer can be captured by the boron during the heat treatment for bonding. As a result, metal contamination in the active layer can be reduced. Moreover, the wafer strength is enhanced, thus preventing the wafer slipping. Since the wafer has no COP, micro voids are not detected in the LPD evaluation of the active layer, thereby improving the reliability of the evaluation. Such a bonded wafer can be manufactured at a low cast.

FIELD OF THE INVENTION

The present invention relates to an SOI wafer and a manufacturing methodof the same, and more specifically to a technology for manufacturing anSOI wafer inexpensively by taking advantage of a bonding method capableof: decreasing a level of contamination resultant from metal impuritiescontained in an active layer; providing a sufficient strength tosuppress an occurrence of slip of a supporting wafer; and improvingprecision in the LPD evaluation of the active layer of thin film.

DESCRIPTION OF THE PRIOR ART

Recently, film thickness reduction of an active layer (less than 0.10 μmhas been progressed in conjunction with a highly densified integrationof devices. There has been developed the smart cut method as a methodfor manufacturing a semiconductor substrate having an SOI (Silicon OnInsulator) structure for achieving the above-mentioned film thicknessreduction.

In the smart cut method, firstly a wafer for active layer, which hasbeen processed to have an oxide film formed thereon and thenion-implanted with hydrogen at a predetermined depth thereof via theoxide film, is bonded with a supporting wafer in a room temperature, andsecondly, thus obtained bonded wafer is introduced into a furnace forheat treatment, where it is heat treated at 500° C. for 30 minutes tothereby cleave and separate a part of the active layer wafer at the siteof the ion-implanted area, which is followed by the step of heattreatment for bonding applied to the bonded wafer in order to enhancethe bonding strength. This can produce a bonded SOI wafer comprising thesupporting wafer and the active layer wafer with a buried silicon oxidefilm intervening therebetween. In the step of the heat treatment forbonding, a heat treatment is applied to the bonded wafer at 1100° C. inan atmospheric gas of oxygen or nitrogen for two hours.

It is to be noticeable concerning this method that a small quantity ofmetal impurities tends to be introduced and mixed in the active layerwafer and the active layer in the process using the high temperature,such as the steps of hydrogen ion implantation and the step of heattreatment for bonding. The metal impurities, such as Fe, Cu, amongothers can permeate through the buried silicon oxide film during theheat treatment after the bonding step and further diffused into thesupporting wafer. In this regard, there is known one method referred toas the IG (Intrinsic Gettering) method, for example, as disclosed in thePatent Document 1, which takes advantage of the above-discussedphenomenon to capture a part of the metal impurities in the supportingwafer.

Further, in recent years, the film thickness has been reduced as thin asless than 0.10 μm (e.g., 0.02 μm to 0.05 μm) for the active layer and0.15 μm for the buried silicon oxide film. Owing to this, the LPD (LightPoint Defect) evaluation for measuring a surface defect (such as COP) ofthe active layer with a particle counter has a fear that a micro voidcould be detected as a pseudo defect. The term, micro void, refers to aminute gap present between the silicon oxide film and the supportingwafer (such as the COP emerging in a bonding interface of the supportingwafer). Such a detection of the micro void as the pseudo defect is dueto the fact that a laser light used for the measurement can pass throughthe active layer of thin film and the buried silicon oxide film. As aresult, the reliability of the LPD evaluation has been made low.

To solve the problem of such pseudo defect, it is contemplated by way ofexample to use a silicon wafer with no presence of COP as disclosed inthe Patent Document 2.

[Patent Document 1]

Japanese Patent Laid-open Publication No. Hei9-326396

[Patent Document 2]

Japanese Patent Laid-open Publication No. Hei 2-267195

SUMMARY OF THE INVENTION Problem to be Solved by the Invention

The invention as disclosed in the above-cited Patent Document 1 hasemployed a silicon wafer of n-type having a specific resistance of 10Ωcm and an oxide concentration of the order of 1×10¹⁸ atoms/cm³ for thesupporting wafer. The prepared wafer is annealed to form an oxidedeposition, and the oxide deposition is then used to capture the metalimpurities including Fe, Cu, which have permeated through the buriedsilicon oxide film, in the region of supporting wafer. Resultantly, thiscan reduce the level of contamination from the metal impurities in theactive layer. The gettering effect as high as that from the depositioncan be obtained alternatively by forming a polycrystal silicon on a backsurface of the supporting wafer and/or by doping of phosphorus (P) ofhigh concentration by the thermal diffusion immediately below the oxidefilm.

However, the heat treatment for bonding involves the heat treatment ashigh as 1100° C. This inversely stimulates the occurrence of slip in thesurface defined in the supporting side of the supporting wafer. In orderto improve this situation, the prior art has employed a method in whichthe bonded wafer is carried by a wafer retaining jig such as an annularsuscepter made of ceramics or a wafer boat having an annular waferretaining portion. However, there has been a problem from the viewpointof cost that the wafer retaining jig is expensive.

In the invention as disclosed in the above-cited Patent Document 2, amonocrystal ingot of silicon is grown by using a rate of 0.8 mm/min orlower for the pulling up in the CZ method. In this case, the growthunder the optimized conditions for the silicon pulling up rate and thethermal environment in the pulling up allows the interstitial siliconthat will be taken in the pulling interface and the vacancy to destroyeach other. Owing to this, the COP, which will be formed from thevacancy being condensed, is no more present and thus the monocrystalingot of silicon free from the COP can be obtained. If the silicon waferwith no presence of COP is used for the supporting wafer, the micro voidwould be no more generated.

However, the pulling up of a wafer at a lower rate requires a long timefor the pulling up as compared to the pulling up of a wafer at such ahigh rate as 1.0 mm/min or higher and produces a lower yield.Consequently, there has been a fear that the manufacturing cost of thebonded SOI wafer will be increased incredibly high.

An object of the present invention is to provide an SOI wafer and itsmanufacturing method which can manufacture such a bonded waferinexpensively by using the smart cut method, in which a level ofcontamination resultant from metal impurities contained in an activelayer can be reduced, the occurrence of slip of the supporting wafer issuppressed and the reliability of the LPD evaluation of the active layerof thin film can be increased.

Another object of the present invention is to provide an SOI wafer andits manufacturing method, in which an autodoping of boron from the backsurface of the supporting wafer can be controlled during the step ofmanufacturing a substrate.

Means to Solve the Problem

A first invention provides a manufacturing method of an SOI wafer,comprising the steps of:

bonding a wafer for active layer with a supporting wafer via aninsulating film interposed therebetween to thereby form a bonded wafer;and then

reducing a film thickness in a part of the active layer wafer of thebonded wafer to thereby form an SOI layer for manufacturing the SOIwafer, wherein

the supporting wafer contains boron by an amount of 9×10¹⁸ atoms/cm³ ormore.

According to the first invention, the metal impurities, such as Fe, Cuthat can diffuse and permeate through the buried insulating film amongthe metal impurities contained in the active layer wafer or the activelayer can be captured by the boron in the supporting wafer via theburied insulating film. Further the Fe, Cu present in the supportingwafer that is movable through the buried insulating film into the activelayer wafer or the active layer can be similarly captured by the boron.It is a matter of course that other metal impurities in the supportingwafer are also captured by the boron. As a result, the level ofcontamination due to the metal impurities in the active layer can bereduced.

Further, the boron as the impurities is present at a high concentrationin the crystal of ingot. This can enhance the strength of the supportingwafer and prevent the supporting wafer from slipping during the heattreatment. The effect of preventing the occurrence of the slip of thesupporting wafer is emphasized in the bonded wafer having the diameterof 300 mm or larger.

Besides, during pulling up of the ingot (e.g., the monocrystal ingot ofsilicon) for the supporting wafer, the crystal contains the boron by anamount of 9×10¹⁸ atoms/cm³ or more. The addition of boron, if reachingto the above-mentioned level, facilitates the OSF (Oxidation InducedStacking Fault) ring within the crystal to contract, without requiringthat the rate of pulling up is set to be a rate as low as 0.5 mm/min,for example. The atomic radius of the boron is smaller than that of thesilicon. When the atom having the atomic radius smaller than that of thesilicon is introduced as the impurities, a tensile stress is generatedaround the impurities, but the presence of interstitial silicon canmitigate the tensile stress and thus reduce the energy in the entiresystem. Specifically, if the boron atom having the small atomic radiusis introduced, an equilibrium concentration of the interstitial siliconis increased, consequently leading to the emergence of the enlarged areaincluding an excessive amount of interstitial silicon or the contract ofthe OSF ring. Generally, the inner side with respect to the OSF ringdefines an area having an excessive number of vacancies, where theabove-mentioned micro void or the COP exists. On the other hand, theouter side with respect to the OSF ring defines an area having anexcessive amount of interstitial silicon, where the COP is not present.Accordingly, it means that the COP is no more existing in themonocrystal ingot of silicon with the OSF ring having contractedcompletely, and if such monocrystal ingot is applied for the SOIsubstrate, the pseudo defect resultant from the COP can be avoided.

Further, the supporting wafer that has been produced by slicing theingot containing the boron by an amount of 9×10¹⁸ atoms/cm³ includes noCOP crystal defect owing to the contraction of the OSF ring as discussedabove. Therefore, for example, even if the active layer and the buriedsilicon oxide film are processed to have their film thickness reduced tosuch a level that a laser light for the LPD evaluation can pass throughthe layer and the film, respectively, the LPD evaluation never detectsthe micro void existing between the buried insulating film and thesupporting wafer as the pseudo defect. Consequently, the reliability ofthe LPD evaluation of the active layer can be enhanced.

More advantageously, the bonded wafer according to the present inventioncan be obtained simply by adding the boron by an amount of 9×10¹⁸atoms/cm³ or higher into the crystal during the pulling up of the ingotfor the supporting wafer, as discussed above. Using the supporting waferthat has been produced by slicing the ingot of high pulling rate allowsthe SOI wafer to be manufactured at a low cost.

The manufacturing method of the SOI wafer according to the presentinvention includes a bonding method, in which the active layer wafer isbonded with the supporting wafer via the insulating film interposedtherebetween to form the bonded wafer, and then the film thickness isreduced in a part of the active layer wafer of the bonded wafer thus toform the SOI layer. One of such methods includes a method in which apart of the active layer wafer of the above-discussed bonded wafer isground and polished to thereby reduce the film thickness formanufacturing the SOI wafer.

The bonding method further includes the smart cut method, in whichfirstly hydrogen gas or a noble gas element is ion-implanted into theactive layer wafer so as to form the ion-implanted layer in the activelayer wafer, secondly said active layer wafer and said supporting waferare bonded together to form the bonded wafer, and then the bonded waferis subjected to the heat treatment as it is held at a predeterminedtemperature so that a part of the active layer wafer can be cleaved andseparated away at the site of the ion-implanted layer as the interface.

The type of the active layer wafer may employ, a monocrystal siliconwafer, a germanium wafer, a silicon carbide wafer and the like, forexample.

The insulating film may employ an oxide film, a nitride film and thelike, for example.

The thickness of the insulating film maybe, for example, no thicker than0.2 μm, preferably in a range of 0.1 μm to 0.2 μm.

The thickness of the active layer may not be limited. For example, thefilm thickness in a range of 1 μm to 50 μm may be employed for theactive layer of thick film. The film thickness in a range of 0.01 μm to1 μm may be employed for the active layer of thin film.

The boron concentration of no greater than 9×10¹⁸ atoms/cm³ (thespecific resistance ρ>10 mΩcm) in the supporting wafer can adverselyreduce the effect of suppressing the occurrence of the slip.Particularly to those SOI wafers having the diameter of 300 mm, theoccurrence of the slip would not be prevented in the bonding process atthe temperature of 1100° C. or higher, unless an expensive ringretaining boat is used. A preferred boron concentration for thesupporting wafer is 1×10¹⁹ atoms/cm³ or higher (ρ≦5 mΩcm).

A second invention provides a manufacturing method of an SOI wafer asdefined in the first invention, further comprising the steps of:

ion-implanting of hydrogen gas or a noble gas element to said activelayer wafer to thereby form an ion-implanted layer in said active layerwafer;

subsequently bonding the active layer wafer and the supporting wafertogether to thereby form a bonded wafer; and then

heat treating the bonded wafer to thereby induce a cleavage in thebonded wafer at the site of the ion-implanted layer as an interface.

The element to be ion-implanted may include, for example, helium (He),neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) and radon (Rn), whichare the noble gas element, in addition to the hydrogen (H). Thoseelements may be provided in a single element or as a component of thechemical compound.

The dose of the hydrogen gas or the noble gas element for the ionimplantation is not limited. For example, the dose may be 2×10¹⁶atoms/cm².

The acceleration voltage used in the ion implantation of the hydrogengas or the noble gas element may be not higher than 50 keV, preferablynot higher than 30 keV and more preferably not higher than 20 keV. Inthe ion implantation of such a light element, the light element can bemore precisely controlled so that the light element can be concentratedin a depth of target by using the lower acceleration voltage.

The heating temperature of the bonded wafer used for the cleavage is400° C. or higher, preferably in a range of 400° C. to 700° C. and morepreferably in a range of 450° C. to 550° C. It is difficult with thetemperature lower than 400° C. to form the bubbles of light element fromthe light element that has been ion-implanted into the active layerwafer. Inversely, with the temperature higher than 700° C., the oxidedeposition will be formed within the active layer and it may deterioratethe properties of devices.

The atmosphere within the furnace during the cleavage may be theatmosphere of non-oxidizing gas (e.g., an inactive gas such as nitrogen,argon). Alternatively, the process may be carried out in vacuumcondition.

The heating time of the bonded wafer for the cleavage may be one minuteor longer, preferably in a range of 10 minutes to 60 minutes. It isdifficult with the heating time shorter than one minute for the lightelement that has been ion-implanted into the bonded wafer to formbubbles.

After the cleavage and separation process, the heat treatment forbonding intended to enhance the strength obtained from the heattreatment for bonding of the active layer wafer with the supportingwafer may be provided. The heat treatment in this process may be carriedout at a heating temperature of 1100° C. for two hours, for example. Theatmospheric gas in the furnace for thermal oxidation may employ theoxygen gas and the like.

The supporting wafer contains a large amount of boron. Owing to this, itis believed that the boron contamination could be induced from thediffusion of the boron into the active layer wafer or the active layerduring the heat treatment after the bonding step. However, theinsulating film comprising the silicon oxide (the buried insulatingfilm), for example, have a higher level of solid solubility incomparison to the silicon. Owing to this fact, the boron in thesupporting wafer is captured by the insulating film provided in thesupporting wafer. As a result, the boron contamination, which otherwisemay occur from the supporting wafer to the active layer, can beinhibited.

A third invention provides a manufacturing method of an SOI wafer asdefined in the first invention, in which a thickness of the SOI layer isless than 0.10 μm.

A fourth invention provides a manufacturing method of an SOI wafer asdefined in the second invention, in which a thickness of the SOI layeris less than 0.10 μm.

According to the third and the fourth inventions, even if the activelayer and the buried silicon oxide film are processed to have their filmthickness reduced to such a thickness of SOI layer as thin as less than0.10 μm that allows a laser light for the LPD evaluation to pass throughthe layer and the film, respectively, the LPD evaluation never detectsthe micro void existing between the buried insulating film and thesupporting wafer as the pseudo defect. Consequently, the reliability ofthe LPD evaluation of the active layer can be enhanced.

A fifth invention provides a manufacturing method of an SOI wafer asdefined in the first invention, in which an insulating film is formed atleast on a surface opposite to a bonding surface of the supporting waferbefore the step of bonding.

A sixth invention provides a manufacturing method of an SOI wafer asdefined in the second invention, in which an insulating film is formedat least on a surface opposite to a bonding surface of the supportingwafer before the step of bonding.

A seventh invention provides a manufacturing method of an SOI wafer asdefined in the third invention, in which an insulating film is formed atleast on a surface opposite to a bonding surface of the supporting waferbefore the step of bonding.

An eighth invention provides a manufacturing method of an SOI wafer asdefined in the fourth invention, in which an insulating film is formedat least on a surface opposite to a bonding surface of the supportingwafer before the step of bonding.

According to either one of the fifth to the eighth inventions, the boronpresent in the vicinity of the back surface of the supporting wafer mayattempt to diffuse outward from the back surface of the wafer during theheat treatment after the bonding step as a result of its heat. It ishowever noticed that the insulating film has been previously formed inthe back surface of the supporting wafer before the bonding, whichserves as the gettering site for the boron. Owing to this, the outwarddiffusion of the boron from the supporting wafer can be suppressed. Thiscan prevent the autodoping resultant from the boron attempting tointrude into the surface of the active layer wafer or into the surfaceof the active layer.

The insulating film provided on the surface of the supporting wafer inopposite to the bonding surface thereof may be an oxide film or anitride film.

The insulating film may employ a silicon oxide film for the supportingwafer comprising a silicon wafer.

The insulating film may be formed exclusively on the back surface of thesupporting wafer. Alternatively, the insulating film may be formed onboth of the top and the back surfaces of the supporting wafer.

The timing of formation of the insulating film is not limited but may beany time before the bonding of the active layer wafer with thesupporting wafer.

The thickness of the insulating film may be in a range of 0.1 μm to 0.5μm, for example. The thickness less than 0.1 μm could not provide theeffect of suppressing the autodoping. In contract, the thickness morethan 0.5 μm requires a longer time for the film deposition and leads toa higher cost. A preferred thickness of the insulating film is in arange of 0.2μm to 0.4 μm.

For example, the oxide film provided on the back surface of the activelayer wafer may be etched by the HF solution after the production of thebonded wafer. Alternatively, the bonded wafer may be shipped with theoxide film remaining thereon.

A ninth invention provides a manufacturing method of an SOI wafer asdefined in either one of the first to the eighth invention, in which thesupporting wafer is subjected to annealing at 1100° C. or higher in areducing gas atmosphere containing hydrogen gas before the step ofbonding.

A tenth invention provides an SOI wafer manufactured by a methodcomprising the steps of:

bonding a wafer for active layer with a supporting wafer via aninsulating film interposed therebetween to thereby form a bonded wafer;and then

reducing a film thickness in a part of the active layer wafer of thebonded wafer to thereby form an SOI layer for manufacturing the SOIwafer, wherein

the supporting wafer that has been bonded contains boron by an amount of9×10¹⁸ atoms/cm³ or more, and the SOI layer has a thickness of less than0.10 μm.

According to the ninth invention, before the step of bonding, thesupporting wafer is subjected to the annealing at a temperature of 1100°C. or higher in the reducing gas atmosphere containing the hydrogen gas.This can facilitate the outward diffusion of the boron present in thevicinity of the top and the back surfaces of the supporting wafer, sothat the boron diffuses to disappear from the top and the back surfacesof the supporting wafer. Consequently, the autodoping of the boron inthe supporting wafer to the surface of the active layer wafer or thesurface of the active layer can be prevented during the heat treatmentafter the step of bonding.

The timing of the annealing is not limited but may be any time beforethe bonding of the active layer wafer with the supporting wafer.

The reducing gas, other than the hydrogen, may include carbon monoxidegas, sulfur dioxide gas and the like, for example.

The annealing temperature lower than 1100° C. leads to a lower diffusioncoefficient of the boron that could not achieve the outward diffusion ofthe boron in the vicinity of the top and the back surfaces of the wafer.A preferred annealing temperature of the bonded wafer is in a range of1100° C. to 1200° C.

The annealing time may be in a range of 0.1 hour to five hours, forexample. A preferred annealing time for the supporting wafer is in arange of 0.1 hour to one hour.

Effect of the Invention

According to the present invention, the supporting wafer contains theboron by an amount of 9×10¹⁸ atoms/cm³ or more. Owing to this, the metalimpurities of Fe, Cu that has diffused outward and resultantly permeatedthrough the buried insulating film among the metal impurities containedin the active layer wafer or the active layer as well as the metalimpurities contained in the supporting wafer can be captured by theboron serving as the gettering site. As a result, the level ofcontamination from the metal impurities in the active layer can bereduced.

Further, since the boron is present in the crystal of ingot at a highconcentration, the strength of the supporting wafer can be increased andthus the occurrence of the slip of the supporting wafer during the heattreatment can be prevented.

In addition, since no crystal defect exists in the supporting wafer,even if the film thickness is reduced in the active layer and thesilicon oxide film, the LPD evaluation would not detect any micro voidspresent between the buried insulating film and the supporting wafer asthe pseudo defects and so the reliability of the LPD evaluation of theactive layer can be improved.

More advantageously, since such a bonded wafer as described above can beobtained simply by adding the boron by an amount of 9×10¹⁸ atoms/cm³ ormore into the crystal at the time of pulling up of the ingot to beprocessed into the supporting wafer, the bonded wafer having the effectsas described above can be manufactured favorably at a lower cost.

Further, since the oxide film has been formed at least on the surfaceopposite to the bonding surface of the supporting wafer before the stepof bonding, this oxide film can help inhibit the boron from diffusingoutward from the back surface of the supporting wafer during the heattreatment after the step of bonding. Consequently, the autodopingresultant from the boron attempting to intrude into the surface of theactive layer wafer or the surface of the active layer can be prevented.

Still further, since the supporting wafer is subjected to the annealingat the temperature of 1100° C. or higher in the reducing gas atmospherecontaining the hydrogen gas before the step of bonding, the boronpresent in the vicinity of the top and the back surfaces of thesupporting wafer can disappear with the aid of the outward diffusionthereof. As a result, the autodoping of the boron into the surface ofthe active layer wafer or the surface of the active layer during theheat treatment after the step of bonding can be prevented, as well.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow sheet showing a manufacturing method of an SOI waferaccording to a first embodiment of the present invention;

FIG. 2 is a flow sheet showing another manufacturing method of an SOIwafer according to the first embodiment of the present invention;

FIG. 3 is an enlarged sectional view of a main part showing a state ofthe LPD evaluation test in a bonded wafer obtained by a manufacturingmethod of an SOI wafer according to the first embodiment of the presentinvention; and

FIG. 4 is an enlarged sectional view of a main part showing a state ofthe LPD evaluation test in a bonded wafer obtained by a manufacturingmethod of an SOI wafer according to the prior art means.

Description of reference numerals 10 Active layer wafer 12a Siliconoxide film (Insulating film) 13 Active layer 14 Hydrogen ion implantedarea (Ion-implanted area) 20 Supporting wafer 30 Bonded wafer

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will now be describedwith reference to the attached drawings.

First Embodiment

Firstly, a monocrystal ingot of silicon of p-type that has been addedwith boron at a low concentration by an amount of 2×10¹⁵ atoms/cm³ ispulled up in the CZ method, as shown in step 101 of FIG. 1. A rate ofpulling up is 1.0 mm/min. After that, the monocrystal ingot of siliconundergoes a series of processing comprising block cutting, slicing,beveling and mirror polishing. Those steps of processing produce a waferto be prepared as an active layer wafer 10 of p-type and mirror-polishedhaving a thickness of 725 μm, a diameter of 200 mm, a face orientationof (100) face, a specific resistance of 10 Ωcm.

On the other hand, a monocrystal ingot of silicon of p-type that hasbeen added with the boron at a high concentration by an amount of 1×10¹⁹atoms/cm³ is pulled up in the Cz method. After that, the monocrystalingot of silicon undergoes a series of processing comprising blockcutting, slicing, beveling and mirror polishing. Those steps ofprocessing produce a wafer to be prepared as a supporting wafer 20 ofp-type and mirror-polished having a thickness of 725 μm, a diameter of200 mm, a face orientation of (100) face, a specific resistance of 8Ωcm.

Following that step, the active layer wafer 10 is introduced into athermal oxidation device, where the thermal oxidation is applied to theactive layer wafer 10 in an oxygen gas atmosphere, as shown in step S102of FIG. 1. This forms a silicon oxide film 12 a having a thickness ofabout 0.15 μm entirely across the exposed surface of the active layerwafer 10. The condition of thermal treatment may be defined by thethermal treatment at 1000° C. for seven hours.

Subsequently, an intermediate current ion implanting device is used toperform the ion implantation of the hydrogen with an accelerationvoltage of 50 keV into the active layer wafer 10 at a predetermineddepth measured from the mirror finished surface thereof. Thus thehydrogen ion implanted area 14 is formed in the active layer wafer 10.The dose used in this step is 5×10¹⁶ atoms/cm².

Subsequently, the active layer wafer 10 and the supporting wafer 20 arebonded together by using the surface of the active layer wafer 10 andthe mirror-polished surface of the supporting wafer as the bondingsurfaces (the superposed surfaces) via the silicon oxide film 12 ainterposed therebetween with a known jig in a vacuum unit, for example,thus to produce the bonded wafer 30, as shown in step S104 of FIG. 1. Inthis step, the active layer wafer 10 and the supporting wafer 20 areconnected together via the silicon oxide film 12 a interposedtherebetween, which silicon oxide film 12 a at the junction between theactive layer wafer 10 and the supporting wafer 20 defines a buriedsilicon oxide film (insulating film) 12 c. Further, in the step ofbonding, the bonding may be performed after a treatment with oxygenplasma (a treatment to activate the surface with the oxygen plasma) inorder to improve the bonding strength and to reduce a defect from thebonding, such as a void.

Then, the bonded wafer 30 is introduced into a thermal treatment devicefor cleavage, though not shown, and heat treated in an atmosphere ofnitrogen gas at a furnace temperature of 500° C., as shown in step S105of FIG. 1. The duration of heat treatment is 30 minutes. As a result ofthis thermal treatment, a part of the active layer wafer 10 is cleavedand separated from the bonded wafer 30 at the site of the hydrogen ionimplanted area 14 while leaving the active layer 13 on the bondinginterface of the supporting wafer 20. It is also possible to reuse thepart of the active layer wafer 10, which has been cleaved off from thebonded wafer 30, as the silicon wafer serving as the supporting wafer 20for the subsequent manufacturing process.

After the cleavage, the heat treatment for bonding is applied to thebonded wafer 30 in a nitrogen gas atmosphere at 1150° C. for two hours,as shown in step S106 of FIG. 1. This treatment can enhance the bondingstrength between the active layer wafer 10 and the supporting wafer 20.

Following the above step, polishing is applied to the surface of theactive layer wafer 13 by using a polishing device, while at the sametime a process of what is called “a sacrificial oxidation” is applied tothe surface of the active layer 13. In the sacrificial oxidation,firstly the bonded wafer 30 is introduced into the furnace for thethermal oxidation, where the bonded wafer 30 is heat treated in theatmosphere of oxidizing gas at 1000° C. for seven hours. This canproduce a 0.15 μm thick silicon oxide film, though not shown, over thesurface of the active layer 13 including the area damaged upon hydrogenion implantation. After that, the surface of the active layer 13 isbrought into contact with the HF cleaning solution to allow the siliconoxide film to dissolve away in the dipping method. In this step, the toplayer portion of the active layer 13 that has been roughened during thecleavage and separation can be removed along with the silicon oxidefilm.

Thus, the bonded SOI wafer 11 has been produced in the smart cut method,as shown in step S107 of FIG. 1.

As discussed above, during pulling up of the monocrystal ingot ofsilicon for the supporting wafer 20, the crystal contains the boron at aconcentration as high as 1×10¹⁹ atoms/cm³. Owing to this, even if thepulling up is carried out at a high rate of 1.0 mm/min rather than usinga lower rate of 0.5 mm/min or lower, the OSF ring within the crystal canbe contracted, as well. Consequently, such an ingot that includes nocrystal defect and thus such a supporting wafer 20 that contains nocrystal defect can be obtained.

This can facilitate the production of the supporting wafer 20 at a highthroughput as well as high yield. Further advantageously, such a wafercan be produced in accordance with the smart cut method known in theart, excluding the point of increasing the concentration of the boron inthe monocrystal ingot of silicon prepared for the supporting wafer 20 inthe first embodiment. As a result, the bonded SOI wafer 11 having theabove described effects can be manufactured at a low cost.

During the heat treatment after the step of bonding, the metalimpurities contained in the supporting wafer 20 in addition to the metalimpurities of Fe, Cu, which can permeate through the buried insulatingfilm by the diffusion, among the metal impurities contained in theactive layer wafer 10 or the active layer 13 can be captured by theboron. As described above, since a large amount of boron exists in thesupporting wafer 20, which can serve as the gettering site, the metalimpurities contained in the supporting wafer 20 in addition to a part ofthe metal impurities contained in the active layer wafer 10 and/or theactive layer 13 can be captured satisfactorily. As a result, the levelof contamination from the metal impurities in the active layer 13 can bereduced.

Further, the boron, or the impurities, is present in the supportingwafer 20 at such a high concentration as discussed above. This canenhance the strength of the supporting wafer 20 and prevent theoccurrence of slip of the supporting wafer 20 during the heat treatment.

In addition, no crystal defect exists in the supporting wafer 20 thatcontains the boron at as high concentration as 9×10¹⁸ atoms/cm³.Therefore, byway of example as shown in FIG. 3, even if the filmthickness is reduced in the active layer 13 and the buried silicon oxidefilm 12 c to such a level that the laser light for the LPD evaluationcan pass through respective layers, no void would be developed betweenthe buried silicon oxide film 12 c and the supporting wafer 20 as hasbeen observed typically in the prior art. Consequently, the detection ofany micro voids 40 as the pseudo defects in the LPD evaluation, as shownin FIG. 4, would no more occur. As a result, the reliability of the LPDevaluation of the active layer 13 can be improved.

Further, as shown in step S204 of FIG. 2, the oxide film 12 b may beformed on the supporting wafer 20 by using a CVD unit, for example,before the step of bonding, so that the boron in the supporting wafer 20can be inhibited from causing the autodoping over the surface of theactive layer wafer 20 or the surface of the active layer 13 during theheat treatment subsequent to the step of the heat treatment for bonding.

Specifically, during the heat treatment following the step of bonding,the heat stimulates the boron to diffuse outward from the back surfaceof the supporting wafer 20. However, the silicon oxide film 12 b hasbeen previously formed on the back surface of the supporting wafer 20,which can serve as the gettering site for the boron. This can inhibitthe outward diffusion of the boron from the back surface of thesupporting wafer 20. As a result, the autodoping resultant from theboron attempting to intrude into the surface of the active layer wafer10 or the surface of the active layer 13 can be prevented.

It is to be noted that the flow sheet for the manufacturing process ofthe SOI wafer shown in FIG. 2 represents that for manufacturing the SOIwafer as defined in FIG. 1 that has been further added with the stepS204.

Further, other means for preventing the autodoping may employ theannealing process, for example, where the supporting wafer 20 issubjected to the annealing at 1100° C. or higher in the hydrogen gasatmosphere before the bonding as shown in step S104 of FIG. 1. It is tobe noted that instead of the hydrogen gas, other reducing gas atmospheremay be used. This can facilitate the outward diffusion of the boron inthe vicinity of the top and the back surfaces of the supporting wafer20, so that the boron contained in the supporting wafer 20 can diffuseoutward to disappear from both of the top and the back surfaces of thewafer. As a result, the autodoping during the heat treatment after thebonding can be prevented.

A report on a result obtained from a comparison and examination withrespect to the bonded SOI wafers produced in accordance with the presentinvention method and the prior art method, respectively, will be hereinpresented specifically on the occurrence of slip in a back surface of asupporting wafer as well as the distribution of the LPD in an activelayer surface in a bonded SOI wafer.

The X-ray topography (XRT) method was employed for an evaluation methodof the slip. Further, to evaluate the distribution of the LPD, a defectevaluation by using a laser device was employed. Table 1 shows theresult. It is to be noticed that the occurrence of the slip in the tableis indicated by using ◯ for the length of slip no longer than 20 mm andX for the length of slip of 20 mm or longer that are observed.

TABLE 1 Boron concentration in Presence of Occurrence Distribution ofLPD in surface of supporting wafer LPD of slip SOI layer Comparative 2 ×10¹⁵/cm³ Present X More in the center of wafer. example 1 (15 Ωcm)Consistent with COP distribution in the supporting wafer. Comparative 4× 10¹⁸/cm³ Present in X More in the center of wafer. example 2 (15 mΩcm)center of Consistent with COP distribution in wafer the supportingwafer. Test example 1 9 × 10¹⁸/cm³ Nil ◯ No particular distributionobserved (10 mΩcm) Test example 2 1 × 10¹⁹/cm³ Nil ◯ No particulardistribution observed (5 mΩcm)

As apparent from Table 1, for both of the test example 1 (the boronconcentration: 9×10¹⁸/cm³) and the test example 2 (the boronconcentration: 2×10¹⁵/cm³) according to the present invention, no slipas long as 20 mm or longer occurred in the back surface of thesupporting wafer and more favorably no particular distribution of LPDwithin the active layer surface were observed. It is clear that theimprovement has been achieved over the comparative example 1 (the boronconcentration: 2×10¹⁵/cm³) and the comparative example 2 (the boronconcentration: 4×10¹⁵/cm³) according to the art.

1. A manufacturing method of an SOI wafer, comprising: preparing asupporting wafer comprising boron in an amount of 9×10¹⁸ atoms/cm³ ormore; forming an insulating film having a thickness of 0.1-0.5 μm on atleast a surface opposite to a bonding surface of said supporting wafer;ion-implanting hydrogen gas or a noble gas element to an active layerwafer to thereby form an ion-implanted layer in said active layer wafer;bonding said active layer wafer with the other surface of saidsupporting wafer via an insulating film interposed therebetween tothereby form a bonded wafer; and then heat treating said bonded wafer tothereby induce cleavage in a portion of said bonded wafer at the site ofthe ion-implanted layer as an interface to thereby form an SOI layerwith said remaining active layer wafer for manufacturing said SOI wafer,wherein a thickness of the SOI layer is less than 0.10 μm.